A launch vehicle such as a rocket requires a tremendous amount of energy to escape Earth's gravity. Thus, a primary goal in the design of a rocket is to maximize the payload that is carried while utilizing the minimum amount of fuel. An efficient methodology that has been widely used in the aerospace industry is a staged rocket. The premise behind a staged rocket is that unneeded mass is jettisoned as soon as possible thereby increasing the payload that can be sent. FIG. 1 is an illustration of a prior art two stage rocket 100. Rocket 100 comprises a first rocket stage 101, a second rocket stage 102, and a fairing 103. Initial acceleration of rocket 100 from standstill requires first rocket stage 101 to have high thrust engines and large propellant tanks to feed these engines. First rocket stage 101 provides thrust to accelerate the entire mass of rocket 100. First rocket stage 101 is separated from second rocket stage 102 and fairing 103 by a separation joint 104. Separation typically occurs at a high altitude where a large engine is no longer needed thereby greatly reducing the mass of rocket 100.
Second rocket stage 102 is enabled after separation from first rocket stage 101 to provide thrust to keep rocket 100 on its intended path. Similar to first rocket stage 101, second rocket stage 102 is no longer needed as it approaches burning its entire fuel load. A separation joint 105 separates second rocket stage 102 from fairing 103. It may also be noted that under some circumstances that fairing 103 is separated from the rocket during first stage 101 burn in order to shed mass as soon as possible and maximize payload to orbit.
FIG. 2 is an illustration of prior art two stage rocket 100 of FIG. 1 showing first rocket stage 101, second rocket stage 102, and fairing 103 separated from one another. Separation of first rocket stage 101 from second rocket stage 102 exposes a rocket engine of rocket stage 102. Fairing 103 is separated exposing a payload 107 that was housed in fairing 103. Although rocket 100 is greatly simplified it illustrates the need for an extremely reliable separation system. A failure in any one of separation joints 104–106 of FIG. 1 would result in a complete failure of the mission at a cost of time, money, and perhaps human life.
Many different types of separation joints have been proven to be extremely reliable in applications similar to that described hereinabove. One type utilizes an explosive device to alter the separation joint from a fastened state to a decoupled state. In general, a separation joint comprises a first and second element. The first and second elements respectively couple to first and second structures that are to be separated under certain conditions. Typically, the first and second elements of the separation joint connect together in a manner where they will not separate under all normal operating conditions other than when the explosive device is detonated. The most prevalent method of holding the separation joint together is to use bolts, rivets, or other mechanical fasteners.
FIG. 3 is a cross-sectional view of a prior art explosive device 300 used in a separation joint. Explosive device 300 comprises a tube 301 and an explosive material 302. In an embodiment of explosive device 300, explosive material 302 is a mild detonation cord. The mild detonation cord is often encased in a sheath that fits within the cavity of tube 301 such that the mild detonation cord is centrally located within tube 301. For example, the sheath may comprise silicone rubber or a shock absorbing/thermally insulating material. Contamination of the field around the separation joint is a critical issue. Tube 301 contains the debris generated from the explosion to prevent contamination of the area near the separation joint. Also, tube 301 is easily formed in a shape for a particular application. For example, a separation joint between two rocket stages is circular in shape, thus tube 301 is circumferentially placed in the joint separating the two rocket stages to provide simultaneously release when detonated.
FIG. 4 is a cross-sectional view of the prior art explosive device 300 of FIG. 4 after detonation. Detonation of explosive material 302 within tube 301 generates gases that radiate radially from the charge. Tube 301 expands under the pressure of the gases generated by the explosion but is designed not to rupture to prevent particle contamination. In an embodiment of explosive device 300, tube 301 is formed of thin walled stainless steel. The rapidly increasing pressure created by the detonation of explosive material 302 causes tube 301 to expand to final state as shown in FIG. 4.
It is this change in volume of tube 301 from FIG. 3 to FIG. 4 after explosive material 302 is ignited that is used to produce a condition where a separation joint separates. As mentioned previously, separation joints are held together with rivets, bolts or other mechanical fasteners. The expansion of tube 301 generates an extremely high force. The force is applied in a manner to shear rivets or fracture elements of the separation joint thereby releasing the joint to separate.
A significant problem with this type of separation joint are the shockwaves that are generated. The shockwaves are coupled to the attached structures of the separation joint. The problem is greatly exacerbated by the release of constrained energy due to the shearing or fracturing of components in the separation process. Shockwaves of up to 5000 g can be coupled to the attached structure. For example, NASA estimates that 45% of all first day spacecraft failures are attributed to damage caused by high dynamic environments. This problem exists today with all new proposed spacecraft designs. Spacecraft are typically ground tested to detect failures using random vibration, acoustic, and shock testing to simulate a launch environment. Perhaps more sensitive to the shockwaves generated by the separation joint is the payload within the spacecraft. The payload is often extremely sensitive or fragile to shock. The cost increases greatly to design components (in the payload) to be more shock resistant. Much of the research is focused on ways to minimize damage to the payload using isolation and damping techniques on the platform on which the payload is mounted.
Accordingly, it is desirable to provide a separation joint that greatly reduces shockwaves transferred to an attached structure when separation occurs. In addition, it is desirable to provide reduce the cost of manufacture and increase the reliability of the separation system. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.